Unlicensed bands (spectrum bands, frequency bands or operating frequency bands) have for long time now been used as an alternative to licensed bands in order to compensate for spectrum scarcity and high licensing costs of the spectrum for mobile communications. Spectrum scarcity refers to the fact that there is lack of spectrum resources for cellular communication systems relative to traffic demand imposed by popular cellular services such as voice, Internet access and other high bit rate data services. The licensed bands for mobile communication especially in the frequency ranges between about 700 MHz to 2 GHz is scarce since the propagation characteristics in this frequency range are favourable for various types of wireless communication notably wireless digital TV, satellite communication, wireless military communication, wireless public safety networks, etc. Furthermore the licensed bands can be used only by an owner of the spectrum. This means that the licensed bands cannot be used by multiple networks at a same location or area. The licensed bands may operate in an open or wide area (e.g. macro network) and therefore also need to meet tight regulatory radio requirements to ensure protection to other licensed carrier frequencies in the same or in other bands, which in turn may also operate in a wide area.
Examples of unlicensed bands are for example the industrial, scientific and medical (ISM) radio transmission bands. Other examples of unlicensed bands are the short range, also denoted local, communication technologies, such as: Bluetooth operating in the 2450 MHz band, HIPERLAN standardized for the 5800 MHz band, and the IEEE 802.11 family widely deployed in the 2450 MHz and 5800 MHz bands.
While these mentioned unlicensed bands and other frequency bands are free to use, certain rules and regulations concerning maximum output power, power density and so called spurious emissions (signal outside a transmitter's assigned channel) still must be followed.
Wireless communications networks normally operate in licensed bands within a certain geographical region. Being able of operating wireless communications networks in unlicensed bands is becoming more and more attractive. Operation in unlicensed bands has for example advantages of increased bandwidth for transmission of user data, and reduced interference in licensed bands due to steering part of data traffic to the unlicensed bands. However, since many wireless communication devices i.e. user equipments, portable computers, Personal Digital Assistants (PDAs) etc. may share a same unlicensed band, interference management is therefore crucial for obtaining acceptable performance while complying with regulatory constraints. Examples of regulatory constrains are maximum output power level, Specific Absorption Rate (SAR) for handheld devices, out of band emission level, spurious emission, unwanted emission level etc. For example, the 3GPP technical report TR 34.925 specifies levels of so called SAR-requirements that make 3GPP devices compliant with regional regulatory requirements. Likewise, the 3GPP technical specification TS 36.101 Section 6.6.2.2 specifies spectrum emission masks that impose limits on 3GPP compliant UE transmit power levels in specific deployment scenarios.
Two examples of well known wireless communications networks are the Wideband Code Division Multiple Access (WCDMA) and the evolved UMTS Terrestrial Radio Access Network (E-UTRAN) (both Frequency Division Duplex (FDD) and Time Division Duplex (TDD)).
Operation in both licensed and unlicensed frequency bands encompasses transmission and reception of information. Hence, operation in frequency bands refers to both transmission and reception of signals. The frequency band is therefore also termed as an operating band or operating frequency band. However, the transmission and reception in frequency bands may take place in a same or in different frequency bands as explained further. In Frequency Division Duplex (FDD) frequency bands (e.g. used in UTRAN FDD and E-UTRAN FDD), uplink and downlink transmissions normally take place on different carrier frequency channels. Therefore, in FDD frequency bands both uplink and downlink transmission can occur simultaneously in time. On the other hand, in Time Division Duplex (TDD) frequency bands (e.g. used in UTRAN TDD and E-UTRAN TDD) uplink and downlink transmissions normally occur on a same carrier frequency channel but in different time slots or sub-frames. A Half Duplex FDD (HD-FDD) which normally may be used in GSM can be regarded as a hybrid scheme where uplink and downlink transmissions are transmitted on different carrier frequencies and which also may be transmitted on different time slots or sub-frames. This means uplink and downlink transmission do not occur simultaneously. Both licensed and unlicensed bands can be FDD, TDD or half duplex FDD.
Smaller communications networks in for example hotspot areas, offices and homes are being deployed in order to be able to increase efficiency and coverage of wireless services that are to be provided to end user devices, such as user equipments. Examples of such smaller communications networks are 3GPP home NodeBs (HNBs) and IEEE femto base stations. Other terms used for similar smaller communications networks are: home base stations, local eNodeBs or home eNodeBs. In principle, the terms femto base stations, home NodeBs, home base station, local eNodeBs or home eNodeBs, all refer to similar types of base stations. An area or location that is served by the home base station may be referred to as a femto cell or home cell. One main difference compared to other classes of base stations, such as macro base stations, is that the home base stations are normally owned by private subscribers (end users), who have the liberty to install the home base station at any location. This makes it possible for the home base stations to be deployed at home or at public/private premises such as shopping malls, office buildings, restaurants etc. Therefore, strict network planning is not possible in case of home base stations deployment compared to macro base stations deployment which are deployed according to some well defined principles.
In general, a home base station operating in WCDMA or in E-UTRAN (FDD, TDD) has a maximum output power (Pf, max, antenna) limited to 20 dBm for non Multiple Input Multiple Output (MIMO) case, and for MIMO, 17 dBm per antenna port in case of two transmit antennas or 14 dBm per antenna port in case of four transmit antennas. This may be generalized according to the equation:Pf, max, antenna=20 dBm−10*log 10(N)′  (1)where N represents a number of transmit antenna ports at the home base station. The maximum output power comprises the power of all downlink transmitted channels including common channels such as common pilot or reference signals, synchronization signals, control channels such as scheduling channels and data channels such as shared data channels etc.
A known major problem with base stations operating in unlicensed frequency bands is interference between different devices that share a frequency band i.e. same bandwidth. It is an important issue to recognize that interference sources in the unlicensed frequency bands are different from interference sources in licensed frequency bands. This is because devices operating in unlicensed frequency bands are typically non cooperative but also because their operations are typically unpredictable i.e. out of control for any deployment or planning procedures. Being non cooperative means that classical Inter-Cell Interference Coordination (ICIC) techniques used in order to reduce inter-cell interference, especially in cell edge regions, which are traditionally used for base stations operating in the licensed frequency bands are not applicable or sufficient for base stations operating in unlicensed frequency bands. Therefore, the interference in the unlicensed frequency bands cannot be made “transparent” to the user equipment i.e. applications in unlicensed frequency bands may, and likely would, perform differently as compared to when running over licensed frequency bands.
Access control mechanisms for home base stations (HNB:s, femto RBs) decide if a given user equipment can or cannot connect to that home base station. Selection of an access control mechanism may have a large impact on performance of the overall wireless communications network, mainly due to a role of the access control mechanism in definition of interference. Different known approaches have been proposed for access control:                Closed access: Only a subset of user equipment, defined by the owner of the home base station may connect to the home base station. This is a model of a so-called Closed Subscriber Group (CSG) definition defined by the 3rd Generation Partnership Project (3GPP);        Open access: All User Equipments (UE) with a certain subscription have the right to make use of the home base station;        Hybrid access: A limited amount of the home base station resources are available to all user equipment, while the remaining resources operate in a CSG fashion.        
There are however several issues that need to be addressed in order to be able to perform dual operation of a home base station i.e. in both licensed and unlicensed frequency bands. The home base station needs to decide which frequency band it should operate in and which access control mechanisms it should employ. This is however problematic, because of the unpredictable interference and its impact on experienced Quality of Service (QoS) on user equipments in the unlicensed frequency bands. On the other hand, operating a home base station in unlicensed frequency bands may reduce the interference caused to the macro base station, i.e. in the macro cell.